More than just a corridor: A suburban river catchment enhances bird functional diversity

Globally, as trends of urbanisation continue to intensify, there has been increasing concern over the impacts of urban expansion on biodiversity and greater attention towards addressing these impacts. Ecological infrastructure such as urban rivers and their catchments may enhance biodiversity, ecological functioning and ecosystem service delivery within cities. Birds are good indicators of urban habitat quality because their ecology is well-studied and they are habitat selective. This study assesses the ecological value of a small urban river catchment in Cape Town, South Africa, in terms of its effect on the taxonomic and functional diversity of birds. 178 bird counts were carried out at 89 sites and 95 species were recorded. The nine functional groups considered were present in equal proportions in the catchment and in the whole of southern Africa, making the catchment a microcosm of the region's avifauna in terms of functional composition. Using RLQ and fourth-corner analyses, we showed that the river was responsible for the occurrence of certain species and functional groups that would not otherwise occur in the suburbs. Nutrient movers, insectivores, scavengers and seed dispersers responded strongly to a gradient of distance from the river and the position on the river. Contrary to the homogenised assemblage that might be expected of an urban area, the catchment contains a taxonomically and functionally diverse bird assemblage. The combination of a river, a heterogeneous urban matrix and an adjacent national park makes this catchment an exemplar of the value of ecological infrastructure for urban biodiversity.